Metal/ceramic composites are important specialized materials that are used in a variety of technical applications, e.g., gas turbine engine valves, pumps and other high-wear components, electrical and electronic connectors, and heat sinks. In addition they are used in a multitude of other applications where there is a need to reduce the weight and cost of a material. These composites are difficult to produce in a uniformly dispersed, intimately mixed composition by state-of-the-art processing methods, such as dry pressing or conventional metal casting, in which the ceramic powder is added to the molten metal prior to pouring. The problem of producing uniform composite materials is especially severe at high loadings of the ceramic component, upwards of about 25 vol %.
Metal/ceramic composites offer several distinct advantages to the materials user for certain applications. For example, the composite can have lower weight and higher elastic modulus than the metal component, and greater toughness than the ceramic component. These composites are frequently composed of a continuous metal phase with a ceramic particulate or fiber or a combination of both as a reinforcement phase. Obtaining uniform distribution of the reinforcement phase represents one of the main objectives in producing these composite materials. Indeed the distribution of the particulate phase has a great effect on the final properties of the composite material.
Various techniques have been employed to produce metal/ceramic composite materials. Techniques such as liquid metal pressure casting of ceramic preforms are considered to be relatively high in cost. In the molten metal mixing process, which is considered to be a relatively low cost technique, ceramic powder is mixed with molten metal to produce wrought products or shape castings. The molten composite has to be stirred continuously prior to and during the casting process in order to maintain the particle suspension and minimize the segregation of the ceramic powder in the mixture. Uniform distribution of ceramic particles is very difficult to achieve due to large differences in specific gravity between the metal and ceramic components (e.g., densities of steel and aluminum oxide powder are approximately 7.8 g/cm.sup.3 and 3.99 g/cm.sup.3, respectively).
The processing behavior of the molten metal exemplified by the rheology, sedimentation, reactivity, and fluidity is also affected by the particulate phase. These factors must be controlled carefully when a molten metal composite is used for shape casting. For example, the presence of ceramic particulates in molten aluminum increases the original viscosity of 10.sup.3 poise substantially and changes the rheology to non-Newtonian.
Invariably the viscosity of molten metal increases as the volume fraction of the reinforcement phase increases and particle size decreases. This effect causes significant mold design limitations for casting complex-shaped parts. Sedimentation due to differences in specific gravity between melt and reinforcement particles is one of the limitations of the shape casting process. The settling rate is greatly affected by the shape, size and volume fraction of the reinforcement particles.
The dry pressing process for making composite parts also suffers from distribution problems due to density differences between ceramic and metal powders. Particle segregation can occur during the blending, die-filling and pressing steps.
The present invention provides readily moldable metal/ceramic composite feedstock compounds suitable for injection molding complex parts that circumvent the problems associated with current state-of-the-art shape forming methods. These compounds overcome the shortcomings of other state-of-the-art shape forming methods by providing a uniform distribution of the reinforcement particles. The molding compounds disclosed herein comprise ferrous and/or nonferrous metal powders and ceramic powders in the form of oxides, carbides, nitrides, borides, silicides or combinations of these powders as reinforcement particles. The volume fraction of the reinforcement particles can vary from 0.1 to 99 vol % depending on the type of application for the composite. Water is used as the liquid carrier, and the metal/ceramic composite feedstock compound can be injection molded at low temperatures (approximately 80 to 90.degree. C.) and low pressures (approximately 500 to 1000 psi) to produce net or near-net shape articles. The unfired (green) molded articles can be dried and sintered according to specific sintering schedules for the composition being used in order to achieve the final desired properties.
Injection molding is recognized as a premier forming method for rapidly producing close tolerance net shape, complex parts in high volume. In Fanelli et al, U.S. Pat. No. 4,734,237, and U.S. patent application Ser. No. 08/869,053, the disclosures of both of which are incorporated herein by reference, processes for successfully molding net shape, complex parts in high volume are described.